1. Field of the Invention
The present invention relates to a method for recording information in an optical recording medium of phase change type.
2. Prior Art
In recent years, highlighted are optical recording media which permit recording at a high density and erasing recorded information for overwriting. The xe2x80x9coverwritablexe2x80x9d optical recording media includes an optical recording medium of phase change type, in which information is recorded by changing the crystalline state of a recording layer with a laser beam and the information is read by detecting a change in reflectance which the recording layer shows due to a change in state.
The optical recording medium of phase change type is of great interest, since overwriting can be made by modulating the intensity of a single laser beam and since the optical system of a drive unit therefor is simple.
When information is recorded in the optical recording medium of phase change type, a laser beam having a power (recording power) to increase the temperature of the recording layer to a melting point thereof or higher is applied to the recording layer. In a region where the recording power is applied, the recording layer is melted and then quenched to form an amorphous record mark. For erasing the record mark, the region of the record mark is irradiated with a laser beam having a power (erasing power) to increase the temperature of the recording layer to a temperature equal to, or higher than, a crystallization temperature thereof but lower than the melting point. When the erasing power is applied to the record mark, the record mark is heated to the crystallization temperature or higher and then allowed to cool gradually, to recover the crystalline state. Modulating the intensity of the single laser beam therefore enables the overwriting.
The method of forming the record mark generally includes two methods; one is a mark position recording method in which information is marked in only a position where a record mark is formed, and the other is a mark edge recording method in which information is marked in two edges of a record mark. The mark edge recording method is essential for high-density recording since it can double the recording density in the mark position recording method.
In the mark edge recording method, positions of the two edges are of significance, so that it is essential to form an accurate record mark. For this reason, the main stream is that a laser beam for recording is applied like pulses. When a laser beam is applied like pulses, the accumulation of heat in an applied region is prevented, and lengthening of the record mark caused by enlargement of the trailing edge of the record mark can be prevented.
As a conventional method of pulse-like application, for example, there is a method in which laser powers are switched between a high power and a low power at predetermined period intervals for a length of time corresponding to, or smaller than, the length of the record mark. In this method, the recording layer is quenched after application of a high power, so that an amorphous state is easily formed. This method is described, for example, in JP-A-8-235588, JP-A-3-283021 and JP-A-2-165420.
There is also proposed a method in which the duration of the first pulse is made to be larger than the duration of any other pulse. This method is described, for example, in JP-A-9-138947, JP-A-8-235587, JP-A-6-12674, JP-A-6-295440 and JP-A-5-62193.
There is also proposed a method in which, for cooling, a laser beam having a power lower than the erasing power is applied before a first pulse and/or after a last pulse. This method is described, for example, in the above JP-A-9-138947 and the above JP-A-6-295440.
According to studies made by the present inventors, however, the following has been found. Even if a recording waveform is made to correspond to a pulse train and if the duration of each pulse is controlled as described in the above Japanese patent gazettes, jitter cannot be fully decreased when a linear velocity is increased. When the linear velocity is increased, the jitter can be decreased by increasing the power of a laser beam. Since, however, a high-power semiconductor laser is difficult to make and is very expensive, it cannot be applied to commercial-based machines and equipment.
Under the circumstances, an object of the present invention is to prevent an increase in jitter without increasing the power of a laser beam when overwriting is made in an optical recording medium of phase change type at a high linear velocity.
The above object is achieved by the following embodiments (1) to (6) according to the present invention.
(1) An optical recording method using a pulse train having at least 1 unit pulse of an upward pulse and a downward pulse continued therefrom as a recording waveform for optical beam modulation in forming 1 record mark by overwriting in an optical recording medium having a recording layer of a phase change type on a substrate, the pulse train including a pulse train having unit pulses of which the number is n (nxe2x89xa74), the optical recording method satisfying;
T(i)H=TmpH (constant value) when 2xe2x89xa6ixe2x89xa6nxe2x88x921,
T(i)L=TmpL (constant value) when 2xe2x89xa6ixe2x89xa6nxe2x88x922,
and
0 less than T(1)L less than TmpL
wherein T(i)H and T(i)L are values obtained by normalizing upward pulse duration and downward pulse duration of a unit pulse in a position i counted from a first unit pulse of the included pulse train on the basis of a base clock width.
(2) An optical recording method according to the above (1), wherein the overwriting is effected at a linear velocity of at least 3.5 m/s of the optical recording medium.
(3) An optical recording method using a pulse train having at least 1 unit pulse of an upward pulse and a downward pulse continued therefrom as a recording waveform for optical beam modulation in forming 1 record mark by overwriting in an optical recording medium having a recording layer of a phase change type on a substrate, the pulse train including a pulse train having unit pulses of which the number is n (nxe2x89xa74), the optical recording method satisfying;
T(i)H=TmpH (constant value) when 2xe2x89xa6ixe2x89xa6nxe2x88x921,
T(i)L=TmpL (constant value) when 2xe2x89xa6ixe2x89xa6nxe2x88x922,
0xe2x89xa6T(n)L less than TmpL,
and
TmpH less than T(n)Hxe2x89xa6TmpH+0.3
wherein T(i)H and T(i)L are values obtained by normalizing upward pulse duration and downward pulse duration of a unit pulse in a position i counted from a first unit pulse of the included pulse train on the basis of a base clock width.
(4) An optical recording method according to the above (3), wherein the overwriting is effected at a linear velocity of at least 3.5 m/s of the optical recording medium.
(5) An optical recording method using a pulse train having at least 1 unit pulse of an upward pulse and a downward pulse continued therefrom as a recording waveform for optical beam modulation in forming 1 record mark by overwriting in an optical recording medium having a recording layer of a phase change type on a substrate, the pulse train including a pulse train having unit pulses of which the number is n (nxe2x89xa74), the optical recording method satisfying;
T(i)H=TmpH (constant value) when 2xe2x89xa6ixe2x89xa6nxe2x88x921,
T(i)L=TmpL (constant value) when 2xe2x89xa6ixe2x89xa6nxe2x88x922,
0xe2x89xa6T(n)L less than TmpL,
and
0.1xe2x89xa6T(nxe2x88x921)L less than TmpL
wherein T(i)H and T(i)L are values obtained by normalizing upward pulse duration and downward pulse duration of a unit pulse in a position i counted from a first unit pulse of the included pulse train on the basis of a base clock width.
(6) An optical recording method according to the above (5), wherein the overwriting is effected at a linear velocity of at least 3.5 m/s of the optical recording medium.